Recently, as a light source of a projection display apparatus (a projector) capable of large screen display, a solid-state light source such as a light emitting diode that can achieve a longer lifetime than a conventional mercury discharge lamp has been receiving attention. An illumination apparatus of such a projection display apparatus is required to provide brighter illumination so as to achieve a high image quality even in a bright room.
Thus, in order to transmit light emitted from the light source to an image display element more efficiently, a reduction of the optical loss in the optical system of the illumination apparatus is being pursued. Moreover, in order to increase the in-plane uniformity of an image that is magnified and projected onto a screen, it is becoming more important to improve the in-plane uniformity of a light beam illuminating the image display element.
To address these issues, a technique by which light radiated from a light source can be efficiently condensed and an illumination light beam can be rendered uniform is proposed in Patent Document 1, for example. FIG. 9 shows a schematic diagram of a conventional optical system described in Patent Document 1.
In this optical system, optically transparent optical blocks 811 to 813 made of glass, acryl, or the like are disposed on the exit end face side of light emitting diodes 801 to 803, which are light sources emitting monochromatic light of three different colors. The optical blocks 811 to 813 each have an exit end having a larger cross-sectional area than that of an entrance end on the side of the light emitting diodes 801 to 803, and the shape of the cross sections is geometrically similar to the shape of an object to be illuminated.
In this configuration, light emitted from the light emitting diodes 801 to 803 first enters the inside of the optical blocks 811 to 813. Subsequently, the light is color-synthesized by a color synthesizing prism 861 or the like, and then illuminates an optical modulator 883, which is the object to be illuminated, via condenser lenses 821 and 881 and a polarization beam splitter 882. The light modulated by the optical modulator 883 is projected onto a screen (not shown) by a projector 884.
Light incident on side faces of the optical blocks 811 to 813 is totally reflected by the side faces within the optical blocks 811 to 813. This is because the optical blocks 811 to 813 are made of a substance having a higher refractive index than the ambient air.
In this case, the number of times of reflection of light ray that enters the optical blocks 811 to 813 at a small angle of incidence within the optical blocks 811 to 813 before the light ray arrives at the exit end of the optical blocks 811 to 813 is small, and the number of times of reflection of a light ray having a large angle of incidence is large.
Accordingly, light arriving at the exit end of the optical blocks 811 to 813 is in a state where light rays that have been reflected different numbers of times are superimposed. Thus, at the exit end of the optical blocks 811 to 813, the uniformity is much improved compared with that at the entrance end face.
An optical element that causes a phenomenon in which light beams are superimposed in this manner is called an integrator. An example of the integrator, such as the optical blocks 811 to 813, is called a tapered rod integrator because such an integrator is an optical component having a tapered shape in which the size at the entrance end and the size at the exit end are different from each other.
Highly uniform light beams exiting from the exit end of the tapered rod integrators 811 to 813 are transmitted in a geometrically similar shape by a lens system disposed between the tapered rod integrators 811 to 813 and the light modulator 883, which is the object to be illuminated, and thus uniformly illuminate the light modulator 883, that is, the object to be illuminated.
Moreover, another technique by which an illumination light beam can be rendered uniform is proposed in Patent Document 2, for example. FIG. 10 shows a schematic diagram of a conventional optical system described in Patent Document 2. In this optical system, light emitted from light emitting diodes 901 to 903 serving as light sources first is collimated by lenses 911 to 913. The collimated wide beams of light are color-synthesized by a three-color synthesizing prism constituted by prisms 961 to 963 and optical thin films 971 and 972.
The color-synthesized light is divided by an optical apparatus called a lens array 933 in which a plurality of lenses are arranged in the same plane. The divided light passes through lenses 941 and 981 and a polarization beam splitter 982, and individual light beams into which the light has been divided are superimposed on a light modulator 983, which is the object to be illuminated, and illuminate the light modulator 983. The light modulated by the light modulator 983 is projected onto a screen (not shown) by a projector 984.
It should be noted that in order to obtain uniformity, typically, the number of lenses within the lens array 933 is about 100 to 200, and a light beam from the light source is divided into 100 to 200 light beams.
At this time, a lens array that is disposed on the light source side is called a first lens array 931, and a lens array that is disposed on the side of the object to be illuminated is called a second lens array 932. The shape of individual first lenses constituting the first lens array 931 is geometrically similar to the shape of the object to be illuminated as is the case with the exit end of the above-described tapered rod integrators. Light beams divided by the individual lenses of the first lens array 931 are each focused onto the object to be illuminated, while being superimposed, by second lenses constituting the second lens array 932 having a corresponding division number. As a result, the optical modulator 983, which is the object to be illuminated, can be uniformly illuminated.
In this manner, even with a configuration in which a lens array is used as the integrator, an optical modulator, which is the object to be illuminated, can be uniformly illuminated.
However, the conventional optical systems as described above have the following problems. In the optical system as shown in FIG. 9, light emitted from the light emitting diodes 801 to 803 enters the respective tapered rod integrators 811 to 813. The light that has entered is reflected within the rod integrators 811 to 813, with the result that the uniformity of the light is improved to some extent, before exiting from the exit apertures. Subsequently, the light that has exited is color-synthesized by the color synthesizing prism 861 or the like so that the optical axes of the light beams radiated from the light sources of the respective colors coincide with one another, and then illuminates the optical modulator 883, which is the object to be illuminated.
At that time, in some cases, unevenness of light emission within the light emitting surface of the individual light emitting diodes 801 to 803 occurs, or there are variations in the light intensity distribution with respect to the angle of light beams radiated from the light emitting diodes 801 to 803.
Furthermore, in order to obtain a larger optical output, a configuration in which plural semiconductor chips, each of which is a light emitting portion of a light emitting diode, are contained within a single package, or a light emitting diode group in which plural packages each containing a single semiconductor chip are arranged side by side may be used as the light source.
Also in these configurations, due to variations in brightness of the semiconductor chips that emit light or due to a gap between the chips or the packages, unevenness of light emission may occur in the light emitting surface of the light source, and due to variations of the individual semiconductor chips, there may be variations in the light intensity distribution with respect to the angle of light radiated from each of the light emitting portions.
At this time, when the difference in the number of times of reflection within the rod integrators 811 to 813 is small for reasons such as the length of each of the tapered rod integrators 811 to 813 being short, the uniformity of exiting light beams becomes insufficient. In this case, the in-plane brightness unevenness of the light beams that illuminate the optical modulator 883 varies from color to color.
Thus, when light radiated from the three light sources 801 to 803 is superimposed, as in the case where a white color is displayed, there is a problem in that the distribution of brightness unevenness on the surface of the optical modulator 883 varies among the three different colors, which causes the in-plane color unevenness when a white color is displayed.
Moreover, the in-plane color unevenness, as described above, at the time when a white color is displayed occurs even when the optical axes of the light emitting diodes 801 to 803, tapered rod integrators 811 to 813 and the color synthesizing prism 861 are slightly misaligned. For this reason, a very high-precision apparatus for adjusting and holding the optical system is needed, and there is a problem in that it also is necessary to address deformation of the holding apparatus caused by the ambient temperature.
Furthermore, to increase the uniformity of light beams exiting from the exit end of the tapered rod integrators 811 to 813 so as to prevent the occurrence of color unevenness as described above, the maximum number of times the light beams are totally reflected by the side faces within the tapered rod integrators 811 to 813 should be as large as possible.
In order to obtain the same uniformity as a common lens array that divides a light beam emitted from a light source into 100 to 200 light beams, it is necessary that the maximum number of times of reflection within a rod integrator is about five to ten, and in order to obtain sufficient uniformity, it often is necessary that the maximum number of times of reflection is more than ten. Therefore, the rod integrator is required to have a long optical path length in the optical axis direction, or in other words, a long optical block is needed.
However, a long tapered rod integrator is expensive. In addition, when the rod integrator is excessively long relative to the cross-sectional area, the rod integrator is elongated, and thus the possibility of breakage or the like increases when the rod integrator is held.
Moreover, in the optical system as shown in FIG. 10, the lenses 911 to 913 that condense light emitted from the light emitting diodes 901 to 903 are required to capture the light emitted from the light emitting diodes 901 to 903 efficiently and allow the light to exit toward the subsequent optical system. At the same time, the lenses 911 to 913 are required to increase the parallelism of the light exiting from the lenses 911 to 913 and reduce the optical loss in the lens array 933 and the optical system thereafter.
In order to condense light emitted from the light emitting diodes 901 to 903 efficiently, it is advantageous that the distance between the light emitting diodes 901 to 903 and the respective lenses 911 to 913 is decreased. On the other hand, in order to increase the parallelism of light exiting from the lenses 911 to 913, it is advantageous that the distance between the light emitting diodes 901 to 903 and the respective lenses 911 to 913 is increased. That is to say, it is difficult to increase the parallelism and to efficiently condense light at the same time.
As described above, in an illumination apparatus that illuminates a predetermined surface to be illuminated with light emitted from a solid-state light source such as a light emitting diode, it is difficult to construct a more efficient and more uniform illumination system using components that are cheap and have a small possibility of breakage when, for example, the components are held.    Patent Document 1: JP 2000-180962 A    Patent Document 2: JP 2004-70018 A